Eric,
Adjusting carbs are scary. It seems like witchcraft. Let's see what I can
guess, based on playing around with the SU carbs for a couple years. One
thing I know how to do is to put them together, so they work pretty good.
As for adjusting them, I'm still learning (and guessing). If anyone on the
list sees a flaw in the following reasoning, please say so, I won't have
hurt feelings!
How do you check for a worn float needle seat?
A guess: disconnect the rubber hose at the bottom of the bowl, and plug the
bottom with your finger. You'll have to adjust the front carb to keep the
engine running on a single carb, in order to keep the fuel pump going. If
there's a problem with the float or float needle, then the bowl will
overflow and not stop the fuel supply. If there is no overflow, then the
bowl is doing it's job of stopping the gas from coming into the bowl when
it's full.
How to check the jet needle?
Get some blueing dye at a tool shop. Coat the needle and let it dry. Then
test dropping the suction piston assembly. If there is contact between the
needle and the jet, and it's only on one side, it indicates that the needle
is out of adjustment. That would allow fuel to continue flowing past the
other side of the needle.
Another possibility is the jet is scored inside from scraping by an old
needle. I just thought of a way to test that. Take the suction dome off
(and the spring). Do like the test above, and adjust the front carb to run
the engine on a single carb. Have someone start the engine, while you hold
the suction piston with the needle down in the jet (oriented the same way
as if the dome was on). Hold that piston absolutely straight! You don't
want to bend the needle. If there is a problem with a worn jet, then fuel
will gush past the needle and piston. If you haven't adjusted the screw to
lean it out (I'm assuming you have tried to adjust it lean all the way),
some fuel will flow.
Another possibility is a failure of the suction chamber. There is an air
hole that allows air to be drawn out of the chamber, creating a low
pressure that draws the piston upwards. If there is crud blocking this air
passage, then you'll get less air and that'll create a rich mixture. Take
everything apart, clean it with carb cleaner and use an air gun to blow out
everything.
Try these guesses. I haven't tested that one where I say you hold the
piston down. Keep a fire extinguisher handy, and a friend to use it on you.
Please do it outdoors!
Fred
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